Coil overheat sensor and control



P 1965 O P. M. HIGGINS ETAL 3,205,871

COIL OVERHEAT SENSOR AND CONTROL Filed May 6, 1963 2 Sheets-Sheet 1 INVENTORS PHILIP HIGGINS RAYMON CZARNECK] BYROBERT *0 55a ATTORNEY Sept. 14, 1965 P. M. HIGGINS ETAL 3,205,871

COIL OVERHEAT SENSOR AND CONTROL 2 Sheets-Sheet 2 Filed May 6, 1963 Al N M 8 m m 4 TsN NNR E E AL v z T m P. MD N W L IY mm Y B R OBERT M. EN ATTORNEY United States Patent COIL OVERHEAT SENSOR AND CONTROL Philip M. Higgins, Chicago, Raymond P. Czarnecki, Elk

Grove Village, and Robert M. Olsen, Villa Park, 111.,

assignors to Vapor Corporation, Chicago, Ill., a corporation of Delaware Filed May 6, 1963, Ser. No. 278,255 2 Claims. (Cl. 122-504) This invention relates in general to a coil overheat sensor and control for detecting overheating of coils in a steam generating boiler. More particularly, the device of the present invention will sense overheating of coils in a boiler where malfunction, such as a plugged coil, cavitating water pump, etc., would cause heat damage to the coils, and will turn off the burner and give an appropriate alarm.

The device of the present inventoin is applicable to coil type boilers or other boilers wherein the steam outlet pipe is a part of the heat transfer surface, i.e., is exposed to conductive combustion gases and radiation from the burner flame.

Heretofore, it has been the practice to sense the steam outlet temperature with a bimetallic thermal switch or other type thermal switch to shut down the burner or the other complete boiler, or alternatively the bimetallic element would serve to reduce the fuel delivery to the burner by mechanically operating a metering valve. In this type of steam temperature limit control, the outlet pipe serves as a material expanding relative to a stem placed within the pipe of a material possessing a lower tem perature coefficient. Thus, the steam is exposed to a steam flow and a portion of the pipe is exposed to combustion gases and radiation from the boiler flame. The straight steam sensing type of unit as above mentioned is effective I only on those coil overheat conditions where the excessive coil temperature is accompanied by a corresponding increase in the steam outlet temperature and when the rate of mass flow of steam to the sensor is suflicient to transfer heat rapidly to the sensor. The sensor above mentioned, utilizing a stem within the pipe, has the expanding pipe partly exposed to combustion gases, partly to the refractory (which could be hot or cold) and to steam flow on the inner surface. The stem is exposed to the steam only. Thus, the steam temperature control responds to some combination of refractory, steam and pipe surface temperature, rather than just to pipe surface t emperature. Moreover, both types above mentioned change settings as a result of high temperature creep of the metals.

It is therefore an object of this invention to provide a sensor for steam boilers that will overcome the above diificulties and which will measure the surface temperature of the steam outlet pipe.

It is a further object of this invention to provide a coil overheat sensor and control wherein the surface temperature of an experimentally determined hot spot location on the steam outlet pipe is measured, and therefore the temperature signal is representative of the surface temperature of the coil.

A still further object of this invention is to provide a coil overheat sensor and control for a steam boiler which employs a thermocouple integral with the steam outlet pipe and a control operated thereby that includes a magnetic amplifier and a transistorized switching circuit to shut down the main burner of the boiler when an overheat condition occurs.

Other features, and advantages of the invention will be which:

FIG. 1 is a somewhat diagrammatic view of a boiler, with parts broken away to show how the sensor of the present invention is mounted within the boiler;

FIG. 2 is a view of steam outlet pipe and illustrating the formation for receiving the thermocouple in accordance with the present invention;

FIG. 3 is a transverse sectional view taken substantially along line 33 of FIG. 2;

FIG. 4 is an enlarged fragmentary view of a portion of the steam outlet pipe, with parts shown in the section and other parts broken away to illustrate the relation between the thermocouple junction and the outlet pipe;

FIG. 5 is a transverse sectional-view taken substantially along line 5-5 of FIG. 4;

FIG. 6 is a view similar to FIG. 4, but showing a modification of the invention;

FIG. 7 is a transverse sectional view taken substan tially along line 77 of FIG. 6; l

FIG. 8 is an electrical schematic diagram of the coil overheat sensor control; and

FIG. 9 is a schematic view of the contacts for th relays.

Referring to the drawings and particularly to FIG. 1, a boiler is diagrammatically illustrated and generally designated by the numeral 10 which may be of any desired shape. The boiler includes an outer wall 11 that is in this case somewhat cylindrical and provided at the rear end with a back cover 12 and at the front end with a front cover 13. A series of water tubes or coils 14 are mounted within the boiler and arranged to receive the combustion gases from a burner 15 arranged in the front cover 13. The water tubes 14 are continuous and include an inlet pipe 17 and an outlet pipe 16 suitably extending through the walls of the boiler.

The coil overheat control of the present invention includes a sensor 18 which is mounted at the rear of the boiler on the outlet pipe, and in a position where it is exposed to convective'combustion gasesand radiation from the burner flame. More accurately, the sensor 18 is mounted in the outlet pipe so that it is exposed to the combustion gases and radiation from the burner flame on the outer surface and to the flow of steam and water on the inner surface. Therefore, the sensor may react to an overheat condition which would occur on another portion of the heat transfer surface of the water tubes.

The positioning of the sensor within the boiler is critical to react properly and detect overheating conditions, and therefore it must be placed at a hot spot location within the boiler. As seen in FIGS. 2-5, a sheettype, closed end, single wire iron constantan thermocouple 19 is arranged within an elongated indentation or dimple 20 formed longitudinally of the outlet pipe 16. As seen in FIG. 1, the indentation 20 extends along the outlet pipe 16 a length from within the boiler to outside of the boiler.

The thermocouple 19 comprises a constantan wire 21 embedded in a high temperature insulation such as magnesium oxide or the like and identified by the numeral 22 which is in turn received in an iron sheath 23. Thus, the iron sheath furnishes one lead of the thermocouple, while the constantan wire furnishes the other lead. The thermocou-ple junction, as indicated by the numeral 24, FIG. 4, is located at the hot spot location along the outlet pipe 16. The iron sheath is suitably brazed or welded, preferably with copper, into the pipe indentation 20 as indicated by the numeral 25 to form a fillet between the sheath and pipe that defines a heat flow path therebetween. This maintains the sheath at a temperature near that of the pipe to prevent scaling and short life of the sheath and to insure that the thermocouple junction is representative of the hot spot pipe temperature. Moreover, the brazing effectively defines the pipe and thermocouple as one piece, that is, the thermocouple is integral with the pipe.

It should be appreciated that thermocouples of other types may be provided to define the sensor 18. Another modification of a thermocouple is shown in FIGS. 6 and 7, wherein a constantanwire 2-7 is welded to the outlet pipe 16 at the end of the groove as indicated by the numeral .27. A suitable high temperature insulation 28, such as ceramic beading, surrounds the wire, and a steel cover 29 is Welded over-theinsulation to the pipe to protect the constantan wire and insulation from overheating, deterioration and exposure to chemical action by combustion gases, and to further keep the cover close to pipe temperature so that the junction temperature will nearly represent-the-pipe surface temperature. The steel cover and the inlet pipe inthis embodiment provides the other lead for the thermocouple arrangement.

Referring now to FIG. 8, an electrical control is shown which serves in combinationwith the thermocouple type surface sensor 18, and is actuated when the coil or water -tube surface temperature exceeds the normal temperature and before a surface temperature is reached which would result in damage to the coils, such as scaling which would reduce the life of the coils. Actuation of thiselectrical control, upon sensing overheating of the water tubes or coils, functions to turn off the burner and give an appropriate alarm.

This electrical control includes a power supply 30' consisting of a power-transformer 31, a rectifier 32, capacitors 33 and 34, and a voltage divider including a variable resistor 35 and a fixed resistor 36. Thepower transformer 31 consists of primary winding 37 that is suitably connected to an alternating current voltage, a first second- ,ary winding 38 and a second secondary winding 39. The

plied to the trigger circuit 42 and to a thermocouple reference voltage network 43.

The thermocouple reference voltage network 43 in cludes fixed resistors 44, 45 and 46, and a variable resistor 47.

The magnetic amplifier 41 comprises a reactor 49,-saturating rectifiers 50, 51, 52 and 53, resistors 54 and 55 and a load resistor 56. The load resistor 56 is shunted 'bya filter capacitor 57 and a rectifier58 which prevents polarity reversal.

The reactor 49 includes a control winding 59 connected in series with the thermocouple 19, and windings 60,

, 61, 62 and 63respectively connected in series with the saturating rectifiers 50, 51, 52 and 53.

The transistorized trigger circuit 42 is of the voltage level-sensing type and includes transistors 64 and 65, resistors 66, 67, 68 and 69, a capacitor 91, and a rectifier 70. A pilot relay 71 is energized by the trigger circuit A'diode rectifier 71a is connected across the pilot relay to protect the transistor 65 from voltage surges when the inductive load of the pilot relay coil is interrupted.

The resistor 68, together with a capacitor 72 defines a pulsing circuit.

Referring to FIG. 9, means is illustrated for interconnecting the control of the boiler with the electrical control of FIG. 8, and includes boiler and alarm signal circuits 73 and 74, respectively, and a relay signal circuit 75. The relay signal circuit 75 includes a relay 76 operatingcontacts 77 in Ihe relay signal circuit 75, contacts 78 in the burner signal circuit 73, and contacts 79 in the pulsing circuit. The pilot relay 71 operates contacts 80 in the relay signal circuit 75, 81 in the alarm signal circuit 74 and 82 in the burner signal circuit 73. All relay contacts in FIGS. 8 and 9 are shown with the relays in deenergized position.

Additional thermocouple channels can be added to the power supply 30, by adding for each channel a separate reference voltage network, a separate magnetic amplifier and a separate trigger circuit and pilot relay. When the channels are to be of the interchangeable plug-in type, a resistor 83 is added to compensate for load changes in the bias voltage when the number of channels is in creased.

In normal operation, the relay 76 is energized from the boiler control by the application of a signal applied to the relay signal circuit 75. Current flows through the normally open contacts 79a of the relay 76, through resistors 47, 44, and to B as indicated at 84. Then a reference voltage appears across resistor 44 opposed in polarity to the thermocouple voltage. This reference voltage produces a current in the control winding which is amplified by the magnetic amplifier 41 and thereafter appearsas a voltage across the load resistor 56.. This voltage is opposed in polarity to the bias voltage across the resistor 35. The transistorized trigger circuit 42 remains energized with transistor 64 in cut off condition and transistor 65 in saturated or conducting condition. Thus the pilot relay 71 is energized closing the contacts 82 to complete the circuit to the burner control that is connected to the burner signal circuit 73. The contact 78 will have been closed together with the contact 77 -uponenergization of the relay 76. The pilot relay when closing contact 82 opens contacts 80 and 81.

When the thermocouple voltage increases to the control setting, corresponding to an overheating condition at the thermocouple sensor, the current in the control winding 53 is reduced thereby resulting in a lower voltage across the load resistor 56. This in turn reduces the voltage input to the trigger circuit 42 to the switching level, and the trigger circuit deenergizes the pilot relay 71. Deenergization of the pilot relay 71 opens contacts 82 and closes contacts 80 and 81, thereby interrupting the power to the burner control, while closing the circuit to the alarm which is connected to the alarm signal circuit 74.

The value of the resistor 67 in the trigger circuit 42 is such as to provide a large voltage differential so as to effect a lock out of the electrical control and require manual resetting before the boiler can again be fired. Therefore the boiler is again fired by first open- I ing a fuel switch on the boiler momentarily and reclosing same to effect deenergization and energization of the relay 76 and pulsing of the trigger circuit 42 as will be hereinafter described.

When the fire from the burner is cycled off by the boiler control, power is removed from the relay signal circuit to thereby deenergize relay 76. The reference voltage across the resistor 44 is reduced since the resistor 46 is no longer shunted by the normally open contact 79a of the relay '76 so that the resistor 46 is then in series with the resistor 47. The current in the control winding is reduced to a value that will cause the trigger circuit 42 to deenergize the pilot relay 71.

Upon deenergization-of the pilot relay 71, the normally closed contacts complete the circuit to the relay 76. Thus, when the boiler control cycles to the fire on condition, power is applied to the relay signal circuit 75 to energize the relay 76. Normally open contacts 79a are then closed so that the reference voltage across the resistor 44 is returned to normal operating level by shunting of the resistor 46. To overcome the large lock out differential of the trigger circuit 42, the capacitor 72 is charged while the relay 76 is deenergized and then discharges through the resistor 68 when the relay 76 is energized. This pulses the trigger circuit 42 by applying a positive voltage to the base of the transistor 64, and thereby causes the trigger circuit 42 to switch on the pilot relay 71. If no overheat condition exists, that is if the thermocouple voltage and the voltage across the resistor 56 is within the normal range, the trigger circuit 42 will hold the pilot relay 71 in the energized condition and the boiler will turn on after a short time delay provided by the boiler control.

As heretofore stated, additional thermocouple channels may be added to the power supply 30 and these channels may be connected in at the points 85, 86, 8'7, 88, 89 and 90. Each channel would include a pilot relay which would have contacts in the burner signal circuit 73, the alarm signal circuit 74 and the relay signal circuit 75. As an example of adding one channel, the additional pilot relay contacts 80a, 81a and 82a would be added as shown particularly in dotted lines in FIG. 9.

1f the pilot relay 71 or any other pilot relay of another channel fails to operate (become deenergized) during the fire off period when relay 76 is deenergized, the relay signal circuit 75 will be interrupted by the normally closed contacts 80 of the energized pilot relay 71 and the signal applied to the relay signal circuit 75 by the boiler control will fail to energize the relay 76. The burner signal circuit 73 is thusly interrupted by the normally open contact 78 of relay 76 and the burner will not turn on.

Upon open circuit of the thermocouple lead wire or failure of the reference supply voltage to the reference resistor 44, the current in the control winding will be reduced to zero (reversed polarity), which will in turn reduce the voltage across the load resistor 56 to approximately zero, since the diode rectifier 58 prevents a reversal of the polarity on the load resistor. The trigger circuit 42 will then deenergize the pilot relay 71 to open the burner signal control circuit 73 and shut off the burner.

Should the reference resistor 44 open circuit, during the safe start check, the current through resistors 45, 46 and 47 when the relay 76 is deenergized will be diverted through the control winding 59 and will maintain the voltage across the load resistor 56 above the switching level. This will maintain the trigger circuit to keep the pilot relay 71 energized so that the normally closed contacts 80 of the pilot relay will be open and will interrupt the relay signal circuit 75 to thereby prevent the energization of the relay 76 which will prevent burner operation and the closing of the burner signal circuit 73.

It should be appreciated that power is applied to the alarm signal circuit 74 by the boiler control only when the boiler control circuit is in the fire on condition so that the alarm circuit will be energized only when an overheat condition occurs.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention, but it is understood that this application is to be limited only by the scope of the appended claims.

The invention is hereby claimed as follows:

1. In a coil type boiler including a set of coils arranged Within boiler walls and having a water inlet pipe, a steam outlet pipe and a burner for imparting heat to said coils, said steam outlet pipe extending through said boiler walls, a coil overheat sensor and control unit comprising, means on said steam outlet pipe defining an elongated dimple along its outer surface extending from within said boiler through said boiler walls and to outside of said boiler, a thermocouple secured in and coextensive with said dimple and being exposed within said boiler to the output of said burner, the junction of the thermocouple being positioned at a hot spot along said steam outlet pipe, means for amplifying the signal of said thermocouple, and electronic triggering means responsive to the amplified thermocouple signal when the coil surface temperature reaches a predetermined level to shut down said burner.

2. In a coil type boiler including a set of coils arranged within boiler walls and having a water inlet pipe, a steam outlet pipe and a burner for imparting heat to said coils, said steam outlet pipe extending through said boiler walls, a coil overheat sensor and control unit comprising, means on said steam outlet pipe defining an elongated dimple along its outer surface extending from within said boiler through said boiler walls and to outside of said boiler, a thermocouple secured in and coextensive with said dimple and being exposed within said boiler to the output of said burner, said thermocouple being a sheath type closed end single wire iron constantan thermocouple having a constantan wire insulated from and extending through an iron sheath, a fillet brazed between the iron sheath and said pipe forming a heat flow path from the sheath to the pipe, means for amplifying the signal of said thermocouple, and electronic triggering means responsive to the amplified thermocouple signal when the coil surface temperature reaches a predetermined level to shut down said burner.

References Cited by the Examiner UNITED STATES PATENTS 2,151,648 3/39 Baker 136-4 2,413,128 12/46 Wills 23614 2,586,998 2/52 Schlenz 122504 X 2,980,334 4/61 Geniesse 236-14 FOREIGN PATENTS 396,477 8/33 Great Britain.

OTHER REFERENCES Industrial and Engineering Chemistry article: A Method of Installing Tube-Wall Themocouples, by E. L. Patton and R. H. Feagan, Jr., pages 823 and 824; vol. 13, No. 11, Nov. 15, 1941.

PERCY L. PATRICK, Primary Examiner.

MEYER PERLIN, Examiner. 

1. IN A COIL TYPE BOILER INCLUDING A SET OF COILS ARRANGED WITHIN BOILER WALLS AND HAVING A WATER INLET PIPE, A STEAM OUTLET PIPE AND A BURNER FOR IMPARTING HEAT TO SAID COILS, SAID STEAM OUTLER PIPE EXTENDING THROUGH SAID BOILER WALLS, A COIL OVERHEAT SENSOR AND CONTRL UNIT COMPRISING, MEANS ON SAID STEAM OUTLET PIPE DEFINING AN ELONGATED DIMPLE ALONG ITS OUTER SURFACE EXTENDING FROM WITHIN SAID BOILER THROUGH SAID BOILER WALLS AND TO OUTSIDE OF SAID BOILER, A THERMOCOUPLE SECURED IN AND COEXTENSIVE WITH SAID DIMPLE AND BEING EXPOSED WITHIN SAID BOILER TO THE OUTPUT OF SAID BURNER, THE JUNCTION OF THE THERMOCOUPLE BEING POSITIONED AT A HOT SPOT ALONG SAID STEAM OUTLET PIPE, MEANS FOR AMPLIFYING THE SIGNAL OF SAID THERMOCOUPLE, AND ELECTRONIC TRIGGERING MEANS RESPONSIVE TO THE AMPLIFIED THERMOCOUPLE SIGNAL WHEN THE COIL SURFACE TEMPERATURE REACHES A PREDETERMINED LEVEL TO SHUT DOWN SAID BURNER. 